Interpretation of Distortion Changes during Endolymphatic Manipulations by an Analysis of Transducer Operating Point.

Davud B. Sirjani , Ruth M.Gill, Alec N. Salt

Abstract

We investigated the relationship between distortion and operating point and utilized its co-dependence for the assessment of the functional state of the cochlea. Three interventions were used to disturb endolymph: 1) 200Hz tone exposure; 2) Artificial endolymph injection at 80, 200 or 400nl/min; 3) Injection of furosemide intravenously or intracochlearly. We measured the effect of volume disturbance on: second harmonic distortion, summating potential, f2-f1 and 2f1-f2 emissions; thresholds at 2.8kHz and 8kHz; cochlear microphonic at 500Hz and 4kHz; and endocochlear potential. Operating point of the transducer was obtained from analysis of the cochlear microphonic waveform using the approach of Kirk and Patuzzi 1997 (Hear. Res. 112, 49). The operating point and second harmonic distortion showed a predictable pattern of change in all intervention groups. Threshold elevations were modest except in the groups receiving furosemide or artificial endolymph injections at a high rate. The f2-f1emissions showed a greater sensitivity to the volume disturbance than did 2f1-f2. The 200Hz tone exposure, 200nl/min artificial endolymph injections and furosemide injection into scala tympani all showed an increase in operating point consistent with an induced acute state of endolymphatic hydrops. The direction of change in second harmonic distortion depended on the pre-treatment operating point value. Initial operating points ranged from +0.08 to -0.35 with the average starting operating point of -0.08. Typically, interventions showed a decrease in distortion if operating point moved towards zero and an increase in distortion if operating point moved away from zero. Second harmonic and f2-f1 distortion levels provide insight to the transducer operating point, compensation mechanisms and perhaps the state of endolymph volume.
 

This study was supported by the National Institutes of Health through the National Institute on Deafness and other Communication Disorders, Grant number DC01368

---

Back to Publications page